1. Field of the Invention
This invention relates to a surface controlled subsurface safety valve for controlling flow within a well. A valve closure element moves to a position permitting subsurface flow when pressurized control fluid is effective upon a valve operator. The invention also relates to a pilot valve for controllably communicating pressurized control fluid to the valve operator and for pressure balancing the valve operator.
2. The Prior Art
A common limitation of present surface controlled subsurface safety values is that a large volume of pressurized control fluid must be displaced against several fluid forces to permit valve closure.
Subsurface safety valves are designed to be failsafe, e.g., normally closed. A closure element is generally urged to a position closing the subsurface flow path by a spring, as disclosed in U.S. Pat. No. 3,696,868, and/or dome pressure chamber, as disclosed in U.S. Pat. No. 3,860,066.
At increasing well depths, the responsiveness of safety valves to decreases in control pressure and the ability of safety valves to close quickly both decrease. For a subsurface safety valve, controlled with a single conduit as closed in U.S. Pat. Nos. 3,233,860; 3,703,193; and 3,860,066, the fluid forces resisting closure are created by a hydrostatic head of fluid within the single conduit. The spring and/or dome pressure chamber must exert a force sufficient to displace control fluid from the control pressure chamber of the safety valve into the single conduit. Displacement of the control fluid is retarded by a hydrostatic pressure force of the fluid in the control conduit, by the inertia of the fluid in the control conduit, and by the friction force developed between the control fluid and the control conduit. Due to space, size, strength of materials, and other design limitations, the ability to swiftly overcome these forces and swiftly displace control fluid from the control pressure chamber with a single spring, multiple spring arrangement, and/or dome pressure chamber is limited.
To counteract the hydrostatic pressure force, some surface controlled subsurface safety valves have a second, balance conduit (See U.S. Pat. No. 3,696,868). Through the balance conduit, pressurized fluid may be communicated to the safety valve both to produce a force counter-acting the force created by the hydrostatic pressure in the control conduit and to assist the spring and/or dome pressure in moving the closure element to a position closing the subsurface flow path. Even with a control conduit and a balance conduit, control fluid must be displaced from the valve operating piston chamber to the control conduit during valve closure. The fluid displacement is still retarded by inertia forces and by the friction forces.
For subsurface safety valves positioned at depths of several thousand feet, the combined forces retarding fluid displacement become quite large.
The speed at which control fluid is displaced from the valve operating piston chamber, dictates the closing speed of the valve closure element. Present surface controlled subsurface safety valves, relying upon spring force and/or dome pressure force, must displace a relatively large volume of fluid against the combined forces retarding fluid displacement created by the fluid in the control conduit. To displace the required volume of pressurized control fluid, and therefore to close the subsurface safety valve, can take as long as one hour. When the time is of the essence for closure of a subsurface safety valve, an hour is simply too long.
The communication of control fluid between the well surface and the subsurface safety valve may include controlled fluid communication through a side pocket receptacle of a side pocket mandrel (See U.S. Pat. No. 3,627,042).
U.S. Pat. No. 4,005,751 discloses utilizing a pilot valve, which is pressure balanced to well fluids, to control communication of control fluid. In a first position of the pilot valve, pressurized control fluid may be effective to open the subsurface safety valve. In a second position of the pilot valve, pressurized control fluid may be effective to close the subsurface safety valve. To both open and close the subsurface safety valve, control fluid pressure is required to exceed well pressure. If the control line is ruptured at the surface, hydrostatic control fluid pressure may not exceed the pressure of flowing well fluids. Under those conditions, a subsurface valve as disclosed in U.S. Pat. No. 4,005,751 may not close.